The present invention is related to an optical decision circuit, based on semiconductor optical amplifiers, which can be used for signal regeneration in optical communication systems. Such a circuit can easily be implemented as an integrated circuit or an OEIC.
In optical long-haul communication systems, signal regeneration at regular distances is a prerequisite because several mechanisms deteriorate the optical signals. Examples of such mechanisms are pure transmission losses and different noise and signal distortion sources in communication components. Therefore there is a common interest in simple high quality optical regeneration circuits which can easily be integrated.
Several signal regeneration mechanisms are known, as illustrated in FIG. 1, to improve the signals at regular distances or times. A generated signal, like e.g. a periodic Return to Zero pulse signal (FIG. 1, a) or any other signal comprising a number of pulses which are at regularly spaced points in time (e.g. :t0, t1, t2), enters an optical fiber. After a certain distance, the signal is deteriorated as illustrated e.g. in FIG. 1, b and needs to be regenerated. The simplest regeneration system is the so-called 1R regeneration (FIG. 1, c), which is in fact an amplification system. The input signal (FIG. 1, b) is amplified in order to bring the signal power level sufficiently above the noise level as illustrated in FIG. 1, c. 
A more advanced system is the so-called 2R regeneration (FIG. 1, d). An optical input signal (FIG. 1, b) which is presented to a 2R regeneration system is set to a fixed high output signal level if the power of the input signal is above a certain threshold level and is set to a fixed low output level if otherwise. This regeneration allows to choose a more optimum decision threshold, i.e. a certain optical power level, at the receivers but in many cases this is still not satisfying.
An even more advanced system is the so-called 3R regeneration (FIG. 1, e). An optical input signal (FIG. 1, b) which is presented to a 3R regeneration system is set to a fixed high output signal level if the power of the input signal is above a certain threshold level and is set to a fixed low output level if otherwise. Furthermore a clock signal (FIG. 1, e), having the same period as the generated signal (FIG. 1, a), is used to retime the input signal pulses in order to coincide exactly with the clock pulses. The retiming allows to remove the signal jitter, i.e. fluctuations in the start instant of the pulses. Since the sampling at the receiver side occurs at instants defined by a periodic clock signal, this time jitter can cause additional detection errors, especially for very high bit rates.
So far, regeneration in optical communication has mainly been 1R regeneration using Erbium-doped fibre amplifiers. 3R regeneration has however been under investigation for several years and has been reported in the literature, e.g. in J. K. Lucek, K. Smith, xe2x80x98All-optical signal regeneratorxe2x80x99, Optics Letters, Vol. 18, pp. 1226-1228, 1993. The clock signal is typically extracted from the signal using a mode locked laser as e.g. in P. B. Hansen et al., xe2x80x98All-optical clock recovery using a mode-locked laserxe2x80x99, El. Lett., Vol. 29, pp. 739-741, 1993, and regeneration is based on a non-linear fibre loop mirror, in which the clock signal is modulated by the signal. This technique however involves a complicated combination of fibre based components, making the regeneration circuit spacious and unsuitable for integration. Besides, it only works for certain modulation formats.
An important part of an optical regeneration system can be a so-called optical decision circuit. An optical decision circuit can be used both for 2R and 3R regeneration. Consequently, for 2R regeneration an optical decision circuit should give a low output power level, i.e. ideally this low level is a zero output power level, if the input power is below a certain threshold value and should give a high output power level, i.e. ideally this high level is a constant predetermined high output power level, if the input power is above the threshold value and there should be a steep transition between the low level and the high level. In a practical implementation, the low level may be small, but different from zero, the high level may slightly vary with input power and the transition between low and high level may be more gradual.
According to the present invention a device with an optical signal at its input and an optical signal at its output for optical signal regeneration is disclosed, where said optical output signal has a predetermined output power level, said device comprising:
a beam splitter for splitting said optical input signal in at least a first and a second signal;
at least a first and a second gain clamped optical amplifier, said first amplifier amplifying said first signal, said second amplifier amplifying said second signal, said first and said second amplifier having a different saturation input power level;
a phase shifting element for shifting the phase of either one of said first signal or said second signal;
a combiner for combining said first and said second signal. Preferably said phase is shifted such that the phase difference between said first signal and said second signal is essentially 180 degrees.
In an embodiment of the invention an optical decision circuit is disclosed, comprising a Mach-Zehnder interferometer (MZI) and two gain clamped semiconductor optical amplifiers (GCSOA""s) and where said interferometer further comprises a splitter, a phase shifting element and a combiner. The GCSOA""s are located in the branches of the interferometer.
In an embodiment of the invention an optical decision circuit is configured such that the beamsplitter for the incident signal as well as the combiner for the output signal are substantially symmetric giving a splitting and combining ratio of 50/50. When the input power of the incident signal is below a certain threshold value the optical decision circuit should return a low output power level, i.e. ideally a zero output power level. In this configuration, the amplitude of a first signal incident on a first GCSOA and the amplitude of a second signal incident on a second GCSOA is the same due to the 50/50 splitting ratio. After amplification of these signals, at least the phase of one of the signals is shifted such that the phase difference between the first signal and the second signal is essentially 180 degrees. Consequently, thereafter combining the first and the second signal using a combiner with a combining ratio of 50/50 is in fact substracting the first and the second signal one from another. Therefore, in order to establish a low output power level the amplification factor for input values below the threshold value should be substantially the same. To accomplish this, preferably, the optical decision circuit should further comprise two essentially identical GCSOA""s, i.e. two GCSOA""s having essentially the same structure, essentially the same dimensions and being composed of essentially the same materials.
In an embodiment of the invention an optical decision circuit is configured such that either the beamsplitter for the incident signal or the combiner for the output signal or the beamsplitter and the combiner are asymmetric giving a splitting ratio of xcex1/(100-xcex1), xcex1 being different from 50 and 0 less than xcex1 less than 100, and/or a combining ratio of xcex2/(100-xcex2), xcex2 being different from 50 and 0 less than xcex2 less than 100. When the input power of the incident signal is below a certain threshold value the optical decision circuit should return a low output power level, i.e. ideally a zero output power level. In this configuration, the amplitude of a first signal incident on a first GCSOA and the amplitude of a second signal incident on a second GCSOA can be substantially different due to the asymmetric splitting ratio. After amplification of these signals, at least the phase of one of the signals is shifted such that the phase difference between the first signal and the second signal is essentially 180 degrees. Consequently, thereafter combining the first and the second signal using a combiner with a combining ratio which can be asymmetric is in fact substracting the first and the second signal one from another. Therefore, this optical decision circuit should comprise two different GCSOA""s, said GCSOA""s should be chosen dependent on the value of the splitting ratio and the value of the combining ratio in order to establish a low output power level of the optical decision circuit for input powers below the threshold value.
In an embodiment of the invention as an alternative an optical decision circuit is disclosed comprising a Michelson interferometer and two essentially identical reflecting GCSOA""s. The interferometer further comprises a 3 dB coupler, being used both as a beam splitter for the incident signal and as a combiner for the output signals of the reflecting GCSOA""s, and a phase shifting element.
In an aspect of the invention an optical decision circuit is disclosed for 3R regeneration, i.e. including retiming. Like for 2R regeneration this optical decision circuit should give a low output power level, i.e. ideally this low level is a zero output power level, if the input power is below a certain threshold value and should give a high output power level, i.e. ideally this high level is a constant predetermined high output power level, if the input power is above the threshold value and there should be a steep transition between the low level and the high level. However by adding a clock signal to the input signal incident on the optical decision circuit, said clock signal having the same period as said input signal, at predetermined times the amplitude of the total signal incident on the optical decision circuit is enhanced resulting in a shift of said transition between said low output power level and said high output power level to thereby establish a regeneration and a retiming of said input signal.
According to the present invention a device with an optical signal at its input and an optical signal at its output for optical signal regeneration is disclosed, where said optical output signal has a predetermined output power level, said device comprising:
a beam splitter for splitting said optical input signal, said optical input signal being a combination of an optical signal incident on said device and a clock signal having the same period as said incident optical signal, in at least a first and a second signal;
at least a first and a second gain clamped optical amplifier, said first amplifier amplifying said first signal, said second amplifier amplifying said second signal, said first and said second amplifier having a different saturation input power level;
a phase shifting element for shifting the phase of either one of said first signal or said second signal;
a combiner for combining said first and said second signal. Preferably said phase is shifted such that the phase difference between said first signal and said second signal is essentially 180 degrees.
In an aspect of the invention a method to achieve regeneration of an optical input signal is disclosed comprising the steps of:
splitting said input signal in at least a first signal and a second signal;
amplifying said first signal by means of a first gain clamped optical amplifier and amplifying said second signal by means of a second gain clamped optical amplifier, said first and said second amplifier having a different saturation input power level;
shifting the phase of either one of said first signal or said second signal;
combining said first and said second signal thereby forming an optical output signal; and
wherein said optical output signal has a predetermined output power level. Particularly said optical output signal has a predetermined high output power level or a low output power level, said low level being substantially below said predetermined high output power level, or an output power level inbetween said high output power level and said low output power level
In another aspect of the invention a method to achieve regeneration of an optical input signal is disclosed comprising the steps of:
combining a clock signal, having the same period as said optical signal, with said optical signal thereby forming an optical signal incident on an optical decision circuit;
splitting said incident signal in at least a first signal and a second signal;
amplifying said first signal by means of a first gain clamped optical amplifier and amplifying said second signal by means of a second gain clamped optical amplifier, said first and said second amplifier having a different saturation input power level;
shifting the phase of either one of said first signal or said second signal;
combining said first and said second signal thereby forming an optical output signal; and
wherein said optical output signal has a predetermined output power level.